Other Answers

There are less comets than there are coins (there's a coin in every world but a small handful of worlds don't have comets) but I imagine you do. Still, if you're willing to try, give it a shot. I wish I could give you a more definite answer. Keep in mind that if you're wrong, you'll have to repeat a large number of stars entirely so that the coin will "count", though.

Okay, I just happened to stumble across someone else's post in another question that implies you need all the medals to get the very last comet of all, meaning after all the bonuses and secrets and everything. That says to me that the others are probably possible without them, but if you want 100%, you'll want to clear them out.